ipv4: Use nlmsg_payload in ipmr file

Leverage the new nlmsg_payload() helper to avoid checking for message
size and then reading the nlmsg data.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250415-nlmsg_v2-v1-7-a1c75d493fd7@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>